Make these modules optional.

This allows compiling this code in Gingerbread.

Bug: 3064326

Change-Id: Idab78a5a31a3c2fc35bcdb773aaecf1e4fb70bcb
diff --git a/Android.mk b/Android.mk
index 4ade77c..7281a76 100644
--- a/Android.mk
+++ b/Android.mk
@@ -53,6 +53,7 @@
 include $(CLEAR_VARS)
 
 LOCAL_MODULE := libprotobuf-java-2.3.0-micro
+LOCAL_MODULE_TAGS := optional
 
 LOCAL_SRC_FILES := $(call all-java-files-under, java/src/main/java/com/google/protobuf/micro)
 
@@ -74,6 +75,7 @@
 include $(CLEAR_VARS)
 
 LOCAL_MODULE := libprotobuf-java-2.3.0-lite
+LOCAL_MODULE_TAGS := optional
 
 LOCAL_SRC_FILES := $(JAVA_LITE_SRC_FILES)
 
@@ -95,6 +97,7 @@
 include $(CLEAR_VARS)
 
 LOCAL_MODULE := libprotobuf-cpp-2.3.0-lite
+LOCAL_MODULE_TAGS := optional
 
 LOCAL_CPP_EXTENSION := .cc
 
@@ -135,6 +138,7 @@
 include $(CLEAR_VARS)
 
 LOCAL_MODULE := libprotobuf-cpp-2.3.0-full
+LOCAL_MODULE_TAGS := optional
 
 LOCAL_CPP_EXTENSION := .cc